1. Field of the Invention
The invention relates to a fuel supply circuit for a turbo-engine such as may be used in the aeronautical industry.
2. Summary of the Prior Art
As shown very diagrammatically in FIG. 1 of the drawings, a conventional fuel supply circuit for a modern turbo-engine usually comprises a low pressure pump 12 fed with fuel via a pipe 10, and a high pressure displacement pump 16 of fixed capacity receiving fuel from the low pressure pump 12 via a pipe 14 and delivering the fuel to the turbo-engine via a pipe 18. The pumps 12 and 16 are driven by the turbo-engine, and the high pressure pump 16 is sized so as to ensure the supply of fuel at the time of heaviest demand, which generally occurs either when starting (subsonic operation of the turbo-engine) or at maximum speed (supersonic operation of the turbo-engine).
Accordingly the high pressure displacement pump 16 of the fuel supply circuit is oversized for all the other phases of flight, and particularly when the rotational speed of the turbo-engine is high and fuel consumption low. These conditions, in which the delivery rate of the high pressure pump 16 may be about 40 times higher than the rate of fuel consumption by the turbo-engine, occur generally in flight at high altitude and low speed.
The excess high pressure fuel delivered by the displacement pump 16 is then returned to the input of the pump via an excess fuel return pipe 22 containing a governer valve 24. On passing through this valve the pressure of the excess fuel flow is reduced from a high pressure to a low pressure by a simple pressure drop. The potential energy lost is converted into heat, and this contributes to the heating of the fuel in the fuel supply circuit.
This phenomenon, which is observed in all cases, is particularly noticeable in twin-spool bypass engines with high compression ratios, and can give rise to increases in the temperature of the fuel in the supply circuit of several tens of degrees. This excessive rise in temperature in the fuel supply circuit becomes a considerable drawback when it is realised that, in an aircraft, the fuel constitutes an efficient and easy means of cooling a great variety of systems.
In U.S. Pat. No. 4 339 917 has ben proposed to use part of the excess fuel available at the delivery of the high pressure displacement pump under certain flight conditions to actuate a turbine, the output shaft of which directly drives the rotor of the low pressure pump disposed upstream of the high pressure pump. In a fuel supply circuit constructed in this manner, part of the potential energy loss at the outlet of the high pressure pump is converted into mechanical energy, and the rise in fuel temperature is therefore lower than in the conventional fuel supply circuits described earlier. However, this solution is still not without drawbacks.
Indeed, since the low pressure pump of the fuel supply circuit will be driven by the turbine only when there is an excess of fuel delivered by the high pressure pump, which is not always the case, particularly on starting, it is necessary to use an oversize high pressure pump so that the low pressure pump is actuated immediately on starting. This oversizing of the high pressure pump results in a temperature increase in the fuel which cancels to a large extent the reduction obtained from the conversion of part of the potential energy available in the excess output of the high pressure pump into mechanical energy.